Boot Loader

From openSUSE

The basic task of any boot loader is to load and start operating system when computer is powered on.

Computer CPU can execute only machine code that is located in ROM or RAM.
RAM is loosing it's content when we power off computer, so after powering on we have only ROM that contains machine code.
Any operating system is too large for the ROM size, so manufacturers put in ROM BIOS (Basic Input Output System) that is small enough to be placed there. When Power-on self-test (POST) is done and hardware initialized, BIOS loads and starts small program (512 B) that has only one purpose to find, load and start bigger, second stage program called boot loader.

The reason for this way around is in fact that BIOS is programmed in factory when it was not known location of operating system on hard disk, so it was developed convention to be on the very beginning of the hard disk, in the first sector. Sector is only 512 B and that is enough to put very small program that will load bigger with full functionality necessary to boot operating system.

Examples of boot loaders are ntldr, GRUB and LILO. The GRUB and LILO are special kind called multiboot loaders, that allows us to select one of few installed operating systems to be used to boot computer.

References

The manual about boot has short overview of Linux boot process. How to find and read manuals is explained article Manual Pages.

See Also

• GRUB
• LILO

External Links

• Booting on Wikipedia is about boot loaders.
• Bootstrapping is about bootstrap process.